Meanwhile, two American aid workers infected in West Africa continue treatments at U.S. hospitals

WEDNESDAY, Sept. 10, 2014 (HealthDay News) -- Acting in response to the devastating Ebola outbreak in four West Africa nations, the Bill and Melinda Gates Foundation announced Wednesday that it has pledged $50 million to help combat the epidemic.

The money will be made available to United Nations agencies and international organizations involved in the response effort. It will enable them and national governments to purchase "needed supplies and scale up emergency operations in affected countries," the foundation said in a news release.

The grant will also be used to develop therapies, vaccines and diagnostic tests to treat patients and prevent further transmission of the often-fatal disease.

"We are working urgently with our partners to identify the most effective ways to help them save lives now and stop transmission of this deadly disease," said Sue Desmond-Hellmann, CEO of the Gates Foundation. "We also want to accelerate the development of treatments, vaccines and diagnostics that can help end this epidemic and prevent future outbreaks."

U.S. and World Health Organization officials have warned that the Ebola virus is spreading faster than health workers in Guinea, Liberia, Nigeria and Sierra Leone can work to contain it.

WHO estimates released Tuesday put the West Africa toll from Ebola at more than 2,200 deaths and 4,200 infections, primarily due to a surge of new cases in Liberia, the Associated Press reported. WHO officials estimate that another 20,000 people could become infected with the virus, which has a mortality rate that can approach 90 percent.

Meanwhile, an American medical missionary being treated at a Nebraska medical center for Ebola infection is showing signs of improvement, according to the hospital.

The patient, Dr. Rick Sacra, 51, of Massachusetts -- the third of four U.S. health-care workers infected by the deadly virus in West Africa -- arrived Friday at the Nebraska Medical Center in Omaha. He is undergoing treatment in the hospital's 10-bed isolation unit.

Doctors there say they are pleased with his progress. He's more alert and responsive, Dr. Phil Smith, director of the Nebraska Biocontainment Patient Care Unit, said in a statement Tuesday, the AP reported.

The physicians "continue to be encouraged by what we're seeing up to this point," Smith said.

The fourth American health-care worker who became infected with Ebola in West Africa continued his second day of treatment Wednesday at Emory University Hospital in Atlanta. The patient, who was airlifted in on Tuesday, has yet to be identified for privacy reasons.

The Atlanta hospital last month successfully treated two other U.S. medical aid workers who had contracted Ebola in Liberia. Dr. Kent Brantly, 33, and Nancy Writebol, 59, were flown in from Liberia in August for aggressive treatment at Emory. Both recovered and are no longer contagious.

Although Emory released no further details about its new patient, the World Health Organization reported Tuesday that a doctor working in an Ebola treatment center in Sierra Leone had tested positive for the disease and was being evacuated, according to the AP.

In Nebraska, Sacra, who's a trained family physician from Massachusetts, is receiving an experimental drug that is different from the one that Brantly and Writebol received, according to news reports. Brantly and Writebol were treated with the novel medication ZMapp, but it's not clear if the drug was key to their recovery.

Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention, said Ebola is primarily being spread in West Africa in two ways: The first is among people caring for people with the disease, whether at home or in health-care settings and hospitals; the second is unsafe burial practices.

Much of the Bill and Melinda Gates Foundation's focus is global health and development. Since its founding in 1997 by Microsoft co-founder Bill Gates and his wife, the foundation has given away $30 billion.